The testing process for integrated circuits requires a means of connecting the electrical leads of the packaged device to the equipment performing the test, so that a test vector of data may be fed into the device and that the result of processing the test vector may be compared with the correct result in order to identify the particular sub-circuit that is not working correctly.
This connection process is most often accomplished using a socket designed to hold the packaged device mounted on a printed circuit board. This board is either directly connected to the tester or is connected through a set of electrical cables. This board is also most often fabricated from multiple levels of metal wiring (usually copper) separated by insulating material. The most common insulating material, among others, is referred to as FR-4.
The basic building block of a multilayer board is a very thin sheet of insulating material plated with copper on one or both sides. Wiring is created by patterning the copper and etching away copper from the areas where there is to be no wiring. This is done on several thin sheets and that are then stacked up with insulating material placed in between. Holes are drilled and connections from level to level are created by plating through the holes.
Despite the fact that many different devices may be placed in the same size and type of package, differing functions of devices require that a specific board be fabricated for each device or chip. An example would be a CPU chip and an ASIC chip, which have very different functions and wiring, each being packaged in a 41×41 land grid array (LGA). Physically each would fit in the same socket, but the connections back to the tester would be completely different. Hence the requirement for a different test board.
When performing failure analysis of defective chips it is often possible to run the device using a reduced number of signals. Regardless of the number of test signals needed, the design and fabrication time for a test board is still the same as for a fully connected board.
The design phase is usually a two to three week process. The fabrication is also a two to three week process. So, in spite of the fact that you may already have a test board with a socket for a 41×41 LGA, the wiring differences cause you to do a redesign and new fabrication. The total time spent is somewhere between six and eight weeks when you add the time spent verifying the design before committing it to hardware, shipping time and inspection of the boards after fabrication.
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